The present invention relates to the casting of iron pipe in a centrifugal casting machine. More specifically, this invention relates to the computer control of the casting process, whereby uniform bell ends of the cast pipe can be formed.
The centrifugal casting of iron pipe is accomplished by the use of a centrifugal casting machine. The machine comprises a rotating mold which is rollable toward and away from an iron trough which is adapted to be inserted into the rotating mold. Molten iron is poured from a ladle into the iron trough and pours out of the end of the trough into the rotating mold. The end of the pipe first formed is the bell end which has a core therein to insure the accurate forming of the bell end of the pipe. However, the core does not extend past the bell end into the length of the pipe. Accordingly, if the casting machine is moved away from the end of the iron trough too soon or too late, the portion of the pipe length adjacent the bell end of the pipe will be either too thin or too thick, and the pipe will be scrap. The rate at which the casting machine mold is moved away from the iron trough end is determined by the design of the casting machine. For the present invention, this rate of movement is assumed to be a manually adjustable constant; once the bell forming time has elapsed, the machine is moved away from the trough to form the length of the pipe. The movement of the casting machine is accomplished by either a hydraulic cylinder, a hydraulic or electrical motor, or a combination of these devices. It is to be understood that in certain casting machines, the casting machine itself remains stationary, and the iron trough is moved away from the casting machine. The principles of the present invention are equally applicable to such an arrangement.
A major problem in the centrifugal casting process used to produce thin wall cast iron pipe is the control of the bell end wall thickness. Variations of parameters associated with both the molten iron such as temperature and the casting machine such as mold condition contribute to the unpredictability of the bell end wall thickness. During the formation of an 8 inch (20.3 cm.) diameter pipe bell end, approximately 80 pounds (36.4 kg.) of iron per second flow into the pipe mold. For a 24 inch (61 cm.) diameter pipe, initial flow rates are about 200 pounds (90.0 kg.) of iron per second. A core is present to form the bell, but the core does not extend into the laying length of the pipe directly adjacent the bell. Consequently, due to the high iron flow rate and the absence of the core in the pipe length, the dwell time of the casting machine in forming the actual bell is critical to wall thickness. Due to the magnitude of the flow parameters and the fact that the tolerances in wall thickness for cast iron water pipe are from 0.04-0.08 inch (0.10-0.20 cm.), it is all but impossible to expect a human to be able to accurately control the casting operation.
Two methods are presently in use to control the dwell time of the casting machine in forming the bell end of the pipe. The manual reverse method has been in use since the invention of the centrifugal casting machine. This method is dependent upon the visual response of the machine operator to determine the changes in the molten iron and casting machine parameters and to start the casting machine rolling away from the pouring trough to form the length of the pipe. As expected, this system results in large variations in pipe wall thicknesses and unacceptable amounts of scrap pipe.
A second method utilizes a timer triggered by an electric eye aimed to sight the molten iron entering the mold. Bell forming dwell time is set by the operator prior to the start of the pouring. The operator's expertise is necessary to set the dwell time according to changes in the iron and machine parameters. This method shows improvement over the manual reverse method, but changes in the pouring cadence, iron control and machine control can contribute to unacceptable results similar to the manual reverse method.
Of course it is the volume of iron which flows during the bell forming time that determines whether the bell end will be properly formed. Attempts to measure this volume of iron and so control the movement of the casting machine have failed due to the destructive nature of the molten iron. Almost any sensing device placed in the iron is destroyed. Further attempts at establishing the iron flow rate by determining the chemical and physical characteristics of the molten iron have proved inaccurate due to the changes in the iron chemistry from batch to batch and in the steadily decreasing temperature of the molten iron. The temperature of the iron trough also affects the iron flow rate. These attempts have failed to produce an accurate pouring control further because they do not provide an analysis of the actual flow or iron being used in the real time sense of the present pipe being formed, but rather usually are based on a calculation of the pouring of the previous pipe.
A problem exists in the centrifugal casting or iron pipe in determining the time period during which the casting machine should not be moved to allow the bell end of the pipe to be formed within allowable tolerances.
Accordingly, it is an object of the present invention to provide an accurately controlled centrifugal pipe casting process.